Electronic system manufacturers must provide electrical power to the fans or blowers needed to cool electronic systems as well as to the system itself. The current trend is to convert primary AC voltage to DC voltage at one location within the system. This DC voltage is then bussed throughout the system. From this DC voltage, DC-AC inverters may provide synthesized AC at any voltage and frequency necessary for the fans, blowers or other load devices.
DC-AC inverters or load controllers used to drive such fans or blowers have gone through a number of design changes. In order to drive three phase AC loads, three pairs of switches are required. Each pair is associated with one load coil and comprises an upper and lower switch device. The switch devices are operated in sequence so that an upper switch device from one pair is operated with a lower switch device from another pair. Various combinations of upper and lower switch devices are sequentially operated to energize successive sets of load coils establishing the field rotation necessary to drive the load.
Operating such three phase load controllers at 100% duty cycle means that as one pair of switches is turned off, another pair is immediately energized. When controlling an inductive load, this insures that any regenerated current from a previously energized load coil can be recirculated through and dissipate in an adjacent, and currently energized coil. To effect rotational speed reductions, it is necessary to reduce the frequency of the synthesized AC. When applying a reduced frequency to the load, it has been found, however, that operating at 100% duty cycle overdrives the load coils, generating excessive heat which may prematurely burnout the coils.
To eliminate overdriving the load coils or achieve power reductions, the controllers are operated at less than 100% duty cycle. The currently selected and energized combination of switch devices is cycled on and off at a very high frequency, typically between 10k to 40k hertz to provide the desired amount of current to the load coils. This technique is commonly called Pulse Width Modulation or PWM. However, the turning off of a pair of switch devices without immediately turning on the next sequential pair eliminates one of the paths through which regenerated current from the previously energized coil may flow. This necessitates providing a shunt path around each switch device to allow for the regenerated current to decay. These shunt paths often consist of large energy storage devices such as capacitors and inductors or large energy dissipative devices such as zener diodes. In addition, the regenerated current can be directed into the power source which must allow for high bursts of currents into its output. Handling these high regenerated current bursts in this manner results in added component costs as well as decreased controller reliability.